Two-Dimensional Stiffness Profiles from SASW Testing at Selected Sites: 1994 Northridge Earthquake

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9416433 Stokoe Several soil and rock sites shaken by the Northridge Earthquake of January 17, 1994 exhibited unusually high ground surface accelerations. The insitu portion of the project is basically one dimensional; that is, only a vertical profile of small-strain material stiffness, in terms of shear wave velocity, will be measured at each site. No investigation of the lateral variability of the stiffness profiles is proposed. The one -dimensional (1-D) approach is taken due to the one year time frame for the project, also because of the high cost involved in installing numerous boreholes that would be necessary to develop two-dimensional (2- D) profiles using borehole seismic techniques. The objective is to develop 2-D (and in one case, 3-D) stiffness profiles. Borehole seismic techniques (such as crosshole testing, downhole testing or suspension logging) will not be employed. Instead, the nonintrusive SASW technique involving Rayleigh-type surface waves is utilized. The technique is nonintrusive because both the source and receivers are placed on the ground surface. Measurement of surface wave dispersion is performed in the field. Forward modeling is then used in the laboratory to determine the shear wave velocity profile at the site. In addition to the cost benefit of being nonintrusive, an important benefit of SASW testing is that it can be used to measure softer layers sandwiched between stiffer layers, a potentially critical measurement in engineering studies. This project involves SASW testing (spectral analysis of surface waves) at two sites: The Tarzana site and the La Cienega and I- 10 highway interchange. In both cases, unusually high ground shaking was measured with surface stations. At each site, in situ seismic testing will be performed using a high-energy mobile vibrator (Vibroseis). This source, coupled with multi-channel recording of four or more 1-Hz geophones, should permit profiles on the order of 100 to 300 m in lateral dimension (depending on the site) to be developed to a depth on the order of 30 to 60 m. In the case of the Tarzana site, profiling will be performed in both the north-south and east-west directions, allowing a 3-D image of the underground shear stiffness to be developed. The results of this study have the potential to offer an important contribution to the understanding of ground shaking levels generated during the 1994 Northridge Earthquake as well as to the development of cost-effective 2-D and 3-D seismic profiling for the prediction of ground response during future earthquakes. ***